Place the butter in a saucepan and heat gently until melted. Add the flour and cook on a low heat, for 1 minute, without colour.
Have the heated Fish stock ready next to the pot with the flour and butter.
Off the heat, gradually, add the Fish stock a small ladle full at a time. Make sure you whisk continuously. This will reduce the risk of any lumps forming. Continue until all the stock has been used.
Return to the heat and bring to the boil, slowly, continuing to stir. Once the sauce has reached boiling point, turn down to a low simmer and cook for 10 minutes stirring occasionally.
If the sauce becomes too thick then you can add a little more stock. If the sauce is too thin, then continue cooking for longer.
When ready to serve, check seasoning and finish with a drop of the truffle oil.
«Add any kinds of soft herbs to finish, squeeze of lemon juice or some flaked fish to really turn this into a lovely starter.»